1. Field of the Invention
The present invention relates generally to a wireless communication system using multi-carrier and, in particular, to a method and apparatus for controlling a power mode of a User Equipment (UE) operating on multiple frequencies.
2. Description of the Related Art
Standardization for a Long Term Evolution (LTE) system, i.e., a next-generation mobile communication system, is underway in the 3rd Generation Partnership Project (3GPP). LTE is a technology for realizing high-speed packet-based communication at a data rate higher than is currently available.
In line with the completion of the LTE standardization, an LTE-Advanced (LTE-A) system is now under discussion, which improves a transfer rate by combining the LTE communication system with several new technologies. One of the representative technologies adopted newly is Carrier Aggregation (CA) based on the use of multiple frequencies.
Unlike the conventional data communication in which a UE uses one downlink carrier and one uplink carrier, carrier aggregation allows a UE to use multiple downlink carriers and multiple uplink carriers. A UE capable of using carrier aggregation technology is referred to as an LTE CA UE.
However, using the LTE CA UE as a multi-frequency-enabled UE has a drawback in that, when one or more carriers is added when using one carrier, a coupling circuit of a Radio Frequency (RF) module and modem in the LTE CA UE consumes electricity. For example, the RF-related module consumes electricity even when there is no traffic flow.
FIG. 1 is a diagram illustrating an LTE system architecture to which various embodiments of the present invention are applied.
Referring to FIG. 1, a radio access network of a mobile communication system includes evolved Node Bs (eNBs) 105, 110, 115, and 120, a Mobility Management Entity (MME) 125, and a Serving-Gateway (S-GW) 130. A User Equipment (UE) 135 connects to an external network via eNBs 105, 110, 115, and 120 and the S-GW 130.
In FIG. 1, the eNBs 105, 110, 115, and 120 correspond to legacy node Bs of a Universal Mobile Telecommunications System (UMTS) system. The eNBs allow the UE 135 to establish a radio channel and are responsible for complicated functions as compared to the legacy node B. In the LTE system, all the user traffic including real time services such as Voice over Internet Protocol (VoIP) are provided through a shared channel and thus there is a need of a device which is located in the eNB to schedule data based on the state information such as buffer states, power headroom states, and channel states of the UEs.
Typically, one eNB controls a plurality of cells. In order to secure the data rate of up to 100 Mbps, the LTE system adopts Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology. Also, the LTE system adopts Adaptive Modulation and Coding (AMC) to determine the modulation scheme and channel coding rate in adaptation to the channel condition of the UE.
The S-GW 130 provides data bearers so as to establish and release data bearers under the control of the MME 125.
The MME 125 is responsible for mobility management of UEs and various control functions. The MME 1235 may be connected to a plurality of eNBs.
FIG. 2 is a diagram illustrating a protocol stack of an LTE system to which various embodiments of the present invention are applied.
Referring to FIG. 2, the protocol stack of the LTE system includes Packet Data Convergence Protocol (PDCP) 205 and 240, Radio Link Control (RLC) 210 and 235, Medium Access Control (MAC) 215 and 230, and Physical Layer (PHY) 220 and 225.
The PDCP 205 and 240 are responsible for IP header compression/decompression, and the RLC 210 and 235 are responsible for segmenting the PDCP Protocol Data Unit (PDU) into segments in appropriate size for Automatic Repeat Request (ARQ) operation.
The MAC 215 and 230 are responsible for establishing connection to a plurality of RLC entities so as to multiplex the RLC PDUs into MAC PDUs and demultiplex the MAC PDUs into RLC PDUs.
The PHY 220 and 225 perform channel coding on the MAC PDU and modulate the MAC PDU into OFDM symbols to transmit over radio channel or perform demodulating and channel-decoding on the received OFDM symbols and deliver the decoded data to the higher layer.
FIG. 3 is a diagram illustrating intra-eNB Carrier Aggregation (CA).
Referring to FIG. 3, an eNB transmits and receives signals through multiple carriers across a plurality of frequency bands. For example, when the eNB 305 is configured to use the downlink carrier 315 with center frequency f1 and the downlink carrier 310 with center frequency f3, the conventional UE receives data on one of the two carriers.
However, the CA-enabled UE (or multi-frequency UE) is capable of transmitting/receiving data on multiple carriers simultaneously. The eNB 305 may allocate extra carriers to the CA-enabled UE 330 to increased data rate of the UE 330 depending on the situation.
Aggregating the downlink carriers or uplink carriers of one eNB is referred to as intra-eNB CA. However, the CA can be implemented by aggregating downlink carriers or uplink carriers of different eNBs.
FIG. 4 is a diagram illustrating inter-eNB CA.
Referring to FIG. 4, the eNB 1 405 uses a carrier with center frequency f1 and the eNB 2 415 uses another carrier with center frequency f2. If the carrier with center frequency f1 and the carrier with center frequency f2 are aggregated for the UE 430, this means that the carriers of two different eNBs are aggregated for one UE.